In a modern internal combustion engine of the common-rail type, a high-pressure pump receives a flow of fuel from a tank by means of a low-pressure pump and feeds the fuel to a common rail hydraulically connected to a plurality of injectors. The pressure of the fuel in the common rail must be constantly controlled according to the engine point either by varying the instantaneous flow rate of the high-pressure pump or by always feeding an excess of fuel to the common rail and by discharging the fuel in excess from the common rail itself by means of a regulation valve. Generally, the solution to vary the instantaneous flow rate of the high-pressure pump is preferred, because it displays a much higher energy efficiency and does not cause overheating of the fuel.
In order to vary the instantaneous flow rate of the high-pressure pump a solution of the type presented in patent application EP0481964A1 or in patent U.S. Pat. No. 6,116,870A1 has been suggested, which describes the use of a variable flow rate, high-pressure pump capable of feeding to the common rail only the amount of fuel needed to maintain the same fuel pressure as the required value within the common rail; specifically, the high-pressure pump is provided with an electromagnetic actuator capable of varying the flow rate of the high-pressure pump instant-by-instant by varying the closing time of an intake valve of the high-pressure pump itself.
Alternatively, to vary the instantaneous flow rate of the high-pressure pump, it has been suggested to insert a flow regulating device upstream of the pumping chamber, comprising a continuously variable section choke, which is controlled according to the pressure required within the common rail.
However, both solutions described above to vary the instantaneous flow rate of the high-pressure pump result mechanically complex and do not allow to regulate the instantaneous flow rate of the high-pressure pump with a high accuracy. Furthermore, the flow rate regulation device comprising a variable section choke displays a small passage section for low flow rates and such small passage section determines a high local pressure loss (local load loss) which may impair the correct operation of an intake valve which regulates the intake of fuel into a pumping chamber of the high-pressure pump.
For this reason, a solution of the type presented in patent application EP1612402A1 has been suggested, which relates to a high-pressure pump comprising a number of pumping elements actuated in reciprocal motion by means of corresponding intake and delivery strokes and wherein each pumping element is provided with a corresponding intake valve in communication with an intake pipe fed by a low-pressure pump; arranged on the intake pipe is a shut-off valve controlled in a choppered manner in synchronism with an initial part of the intake step of each pumping element. In other words, the shut-off valve is a valve of the open/closed (on/off) type which is driven by modifying the relation between the opening intervals and the closing intervals for varying the instantaneous flow rate of the high-pressure pump. In this way, the shut-off valve always displays an efficaciously wide passage section which does not determine an considerable local pressure loss (local load loss).
However, the shut-off valves known until now have a very unfavourable performance/cost ratio and accordingly imply an unjustified cost increase of the high-pressure pump.
DE102004052818A1 discloses a fuel injection system for an internal combustion engine and having an electromagnet for operating the control valve.